Such drives are used more particularly in mine hoists, and are often used in sets of two motors which drive shafts for one or more hoists. The hoists are normally operating synchronously, with one hoist being lowered as the other is raised, and the hoists are driven at the same speeds so that they arrive at their upper and lower destinations simultaneously. This method of controlling mine hoists is used particularly for rock hoisting.
The method suffers from a number of disadvantages. The hoist which is being raised requires a surge of current to the motor to accelerate it to the raising speed, whereas the hoist being lowered requires minimal acceleration and only draws a small current. Nevertheless the two currents occur at the same time and cause a considerable peak of current which is drawn from the main supply, and which causes a high power demand with resulting higher capital investment in the power supply network as well as higher electricity costs.
In addition thyristor converters used in the drives must be rated for a higher voltage in order to match the maximum permissible output voltage required during the regenerating mode which is lower than the maximum permissible voltage during the motoring mode. This results in a limited utilization of thyristor converter capacity during motoring mode which causes power factor reduction and necessitates the application of higher voltage rated thyristor converters.
Furthermore, the mechanical brake systems used in the hoists should be capable of handling high energy dissipation experienced during braking of the load being lowered at a relatively high speed.